Implantable medical devices (IMDs), including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with an external device (ED) via a radio-frequency telemetry link. One such external device is an external programmer used to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data that may typically be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations. An external device may also be a remote monitoring unit which collects data from the implantable device and transmits it over a network to a data gathering center.
External programmers are commonly configured to communicate with an IMD over an inductive link. Coil antennas in the external programmer and the IMD are inductively coupled so that data can be transmitted by modulating a carrier waveform which corresponds to the resonant frequency of the two coupled coils. An inductive link is a short-range communications channel requiring that the coil antenna of the external device be in close proximity to the IMD, typically within a few inches. Other types of telemetry systems may utilize far-field radio-frequency (RF) electromagnetic radiation to enable communications between an MD and an ED over a wireless medium. Such long-range RF telemetry allows the IMD to communicate with an ED, such as an external programmer or remote monitor, without the need for close proximity.
Communications via far-field RF telemetry, however, can be hindered by the effects of multi-path distortion which result in nulls in the transmission pattern of either the external device or the implantable device. In a typical environment, reflections of a transmitted wave caused by walls and other objects result in a standing wave pattern. Areas where the standing wave pattern results in a low amplitude signal below the noise floor are referred to as nulls or null areas. When an external programmer antenna is in a null with respect to an implantable device antenna, the RF link is lost and further communications are not possible. Because a patient may typically be moving around during telemetry sessions, such nulls may be transient and of short duration. Even short-duration nulls, however, cause difficulties when collecting certain types of data from an implantable device such as real-time electrograms.